The kind of ligature you are requesting, Adam, would be very bad for your
language in the long run, because you'd never know whether something was
spelled <ch> and when <c><h>. Unicode doesn't choose to code these things
for that reason. Many of the digraphs already there are either from legacy
character sets (such as a lot of Arabic presentation forms as far as I
know) or are for specific odd practices, like those Croatian digraphs which
are there only to give one to one transliteration to Serbian.

Sometimes there may be a real advantage in processing of a ligature is
encoded, even when it is canonically equivalent to a string of other
characters. Mark Shoulson and I believe this is true about the HEBREW
TETRAGRAMMATON. Apparently some Hangul processing algorithms work better
with precomposed syllables (though historical syllables have to be
processed using a different model).

Welsh and Spanish and Irish and English all use the digraph <c><h> to
represent a single sound just as Slovak does. Welsh and Spanish sort them
as separate letters too. But it would be bad to encode <ch>.